My multimeter may have been destroyed by electricity

[watdahel]

I was following this tutorial about checking the polarity on the 3 prong receptacle on my wall. Eventually it tells me to connect the common probe of my multimeter to ground and the positive probe to the neutral and positive holes alternately in the receptacle to make readings. To my surprise there was an arc and the positive probe was melted leaving my receptacle covered in black. Was I not supposed to do this? Why did this happen? Isn't the neutral and gound wire connected to the same water pipe? My multimeter appears to be still working. I connected the positive probe to the 20 amps on the unit before making the catastrophic reading.

 

[Modelworks]

Originally posted by: erwin1978
I was following this tutorial about checking the polarity on the 3 prong receptacle on my wall. Eventually it tells me to connect the common probe of my multimeter to ground and the positive probe to the neutral and positive holes alternately in the receptacle to make readings. To my surprise there was an arc and the positive probe was melted leaving my receptacle covered in black. Was I not supposed to do this? Why did this happen? Isn't the neutral and gound wire connected to the same water pipe? My multimeter appears to be still working. I connected the positive probe to the 20 amps on the unit before making the catastrophic reading.

Look closely at the meter, is there 3 holes on the front for the probes ? does where you connected the probe read something like 20A max or not fused ?

If so then what you did was like connecting a wire between hot and neutral, a short. Meters usually have 3 holes for probes. One is for ground. One if for measuring voltage. One is for measuring large currents.

When measuring current you place the red probe in the current hole on the meter. Then you connect things so that something like a lamp has one wire that goes directly to the hot side of the AC outlet and the other lamp wire connects to the black wire on the meter. The red wire on the meter then goes to the wall outlet.

Inside the meter between those two holes is a solid piece of wire so it acts as a short. Which is why you got an arc when you inserted the probes.

My meter will beep constantly if you have the meter at any other setting than Amps measurement and have a probe in the current measuring hole. Because if I had it in voltage mode and used the holes it would arc like yours did.

If it arced between the ground and neutral then you probably have a reversed neutral and hot wire on that outlet and were actually connecting ground + hot.

 

[TuxDave]

Heh, feels like my intro to circuits lab I had to teach. I had to reemphasize how to use a multimeter to measure voltage and current and why for the first couple weeks of the course, yet people still managed to short my power supplies by trying to measure the voltage across the terminals in current mode.

 

[bobsmith1492]

Originally posted by: TuxDave
Heh, feels like my intro to circuits lab I had to teach. I had to reemphasize how to use a multimeter to measure voltage and current and why for the first couple weeks of the course, yet people still managed to short my power supplies by trying to measure the voltage across the terminals in current mode.

That's why you use current-limited power supplies!

You voltmeter should be fine; they usually have a fuse somewhere in the back by the batteries. Replace it and it should be able to measure current again.

 

[PottedMeat]

Originally posted by: bobsmith1492

Originally posted by: TuxDave
Heh, feels like my intro to circuits lab I had to teach. I had to reemphasize how to use a multimeter to measure voltage and current and why for the first couple weeks of the course, yet people still managed to short my power supplies by trying to measure the voltage across the terminals in current mode.

That's why you use current-limited power supplies!

You voltmeter should be fine; they usually have a fuse somewhere in the back by the batteries. Replace it and it should be able to measure current again.

Or teach the students how to use them . We had all these expensive HP/Agilent instruments in the lab - scopes, power supplies, etc. and very few of the TAs knew how to use them. Lab exercises took forever and ICs were burning out all over the place.

I believe all the multimeters in my Intro to EE lab had busted fuses. The TA had a big bag ready to go.

 

[TuxDave]

Originally posted by: bobsmith1492

Originally posted by: TuxDave
Heh, feels like my intro to circuits lab I had to teach. I had to reemphasize how to use a multimeter to measure voltage and current and why for the first couple weeks of the course, yet people still managed to short my power supplies by trying to measure the voltage across the terminals in current mode.

That's why you use current-limited power supplies!

You voltmeter should be fine; they usually have a fuse somewhere in the back by the batteries. Replace it and it should be able to measure current again.

Luckily I think they were. I loved it when the students when measuring currents were like "I got 20 amps... is that right?" On top of that I'm pretty sure I burned my hand on some of the chips that were wired wrong when they asked me for help on debugging their circuit board. Ah, fun times.

 

[NeoPTLD]

The 1kV 10kA IR fast acting fuse in my Fluke cost as much as three to four whole cheap HFT multimeters.

That fuse is totally an overkill though. There is no reason to have such a high rating fuse to use at home or in an electronics lab.

That fuse is meant to be able to protect you in case you accidentally try to measure the current available across a 480v service entry busbars without blowing up, but it will also melt if you try to measure the current across a battery too, where a 32v fuse would suffice.

 

[PottedMeat]

Originally posted by: NeoPTLD
The 1kV 10kA IR fast acting fuse in my Fluke cost as much as three to four whole cheap HFT multimeters.

That fuse is totally an overkill though. There is no reason to have such a high rating fuse to use at home or in an electronics lab.

That fuse is meant to be able to protect you in case you accidentally try to measure the current available across a 480v service entry busbars without blowing up, but it will also melt if you try to measure the current across a battery too, where a 32v fuse would suffice.

I stupidly fried the fuse in my Fluke 112 and thought 'they can't be too expensive'. Then I saw it was $10 for a 20kA 1kV fuse. I just soldered a 5A fast fuse onto the old one - just gotta remember its a 5A limit now...

 

[watdahel]

Originally posted by: Modelworks

Originally posted by: erwin1978
I was following this tutorial about checking the polarity on the 3 prong receptacle on my wall. Eventually it tells me to connect the common probe of my multimeter to ground and the positive probe to the neutral and positive holes alternately in the receptacle to make readings. To my surprise there was an arc and the positive probe was melted leaving my receptacle covered in black. Was I not supposed to do this? Why did this happen? Isn't the neutral and gound wire connected to the same water pipe? My multimeter appears to be still working. I connected the positive probe to the 20 amps on the unit before making the catastrophic reading.

Look closely at the meter, is there 3 holes on the front for the probes ? does where you connected the probe read something like 20A max or not fused ?

If so then what you did was like connecting a wire between hot and neutral, a short. Meters usually have 3 holes for probes. One is for ground. One if for measuring voltage. One is for measuring large currents.

When measuring current you place the red probe in the current hole on the meter. Then you connect things so that something like a lamp has one wire that goes directly to the hot side of the AC outlet and the other lamp wire connects to the black wire on the meter. The red wire on the meter then goes to the wall outlet.

Inside the meter between those two holes is a solid piece of wire so it acts as a short. Which is why you got an arc when you inserted the probes.

My meter will beep constantly if you have the meter at any other setting than Amps measurement and have a probe in the current measuring hole. Because if I had it in voltage mode and used the holes it would arc like yours did.

If it arced between the ground and neutral then you probably have a reversed neutral and hot wire on that outlet and were actually connecting ground + hot.

Yah I had the red probe connected to the 20A fused hole and I had the dial at AC voltage setting. My meter is not as special as yours. The only warning it gives is the arcing sound and a destroyed probe. So the 20A hole is only for current. I can't find the manual at the moment.

 

[bobsmith1492]

Originally posted by: TuxDave

Originally posted by: bobsmith1492

Originally posted by: TuxDave
Heh, feels like my intro to circuits lab I had to teach. I had to reemphasize how to use a multimeter to measure voltage and current and why for the first couple weeks of the course, yet people still managed to short my power supplies by trying to measure the voltage across the terminals in current mode.

That's why you use current-limited power supplies!

You voltmeter should be fine; they usually have a fuse somewhere in the back by the batteries. Replace it and it should be able to measure current again.

Luckily I think they were. I loved it when the students when measuring currents were like "I got 20 amps... is that right?" On top of that I'm pretty sure I burned my hand on some of the chips that were wired wrong when they asked me for help on debugging their circuit board. Ah, fun times.

Having just taken intro to circuits a few years back I feel your pain. I'd been building circuits since I was six so I knew what was happening; some of the guys, though, had apparently never built anything in their lives. It was fun though.

You could have some fun with them... "Sure, 20A from a 7400 sounds about right."

 

[NeoPTLD]

Originally posted by: PottedMeat

Originally posted by: NeoPTLD
The 1kV 10kA IR fast acting fuse in my Fluke cost as much as three to four whole cheap HFT multimeters.

That fuse is totally an overkill though. There is no reason to have such a high rating fuse to use at home or in an electronics lab.

That fuse is meant to be able to protect you in case you accidentally try to measure the current available across a 480v service entry busbars without blowing up, but it will also melt if you try to measure the current across a battery too, where a 32v fuse would suffice.

I stupidly fried the fuse in my Fluke 112 and thought 'they can't be too expensive'. Then I saw it was $10 for a 20kA 1kV fuse. I just soldered a 5A fast fuse onto the old one - just gotta remember its a 5A limit now...

The fuse in my 87 (which I have removed and set aside) is 11A, AC/DC 20kA 1kV rated. The one I'm using is the same shape, but it's 250v AC750A 125v AC10kA rated.